The Icosahedral Nonhydrostatic (ICON) model: formulation of the dynamical core and physics-dynamics coupling
Seminar Room 1, Newton Institute
The nonhydrostatic dynamical core of the ICON model is formulated on an icosahedral-triangular C-grid and uses the edge-normal wind component, the vertical wind speed, density and virtual potential temperature as prognostic variables. In the vertical, a generalized terrain-following coordinate based on height is used that allows for a rapid decay with height of topographic structures, thereby reducing the numerical discretization errors over steep mountains. Moreover, a truly horizontal formulation of the horizontal pressure gradient term is used that greatly improves numerical stability over steep mountains. The spatial discretizations are second-order, with some slight degradation near the pentagon points of the basic icosahedron, and are primarily optimized for computational efficiency, ensuring strict mass conservation and consistent tracer transport but no exact conservation of energy or vorticity-related quantities. Time integration is based on a second-order predictor-corrector scheme that is fully explicit in the horizontal and implicit for the terms entering into vertical sound wave propagation. The dynamics time step is therefore limited by the CFL stability condition for horizontal sound wave propagation, but a longer time step (typically 4x or 5x) is used for tracer transport and fast-physics parameterizations. A important aspect of physics-dynamics coupling is that all terms related to latent heat release have to be converted into temperature changes at constant volume because density is used as a prognostic variable and is kept fixed during the physics call.